Recovery of oat starch has been earlier presented mainly as a by-product in the preparation of protein or fibre concentrates. In most of these methods, separation is not presented further than until oat flour, or for the removal of starch for enriching the main products. Thus the U.S. Pat. No. 4,028,468 describes a process, where oat gum, starch and protein are separated from the bran by wet milling in sodium carbonate solution. Starch is separated by an effective centrifugation of the viscous solution, but any further separation of starch nor the purity of the recovered starch is given. The granule size of starch is said to be 5 to 10 .mu.m. Similarly, the Canadian Patent 1,133,446 describes separation of the endosperm from milled oats, but purification of starch only for testing purposes using laboratory methods. U.S. Pat. No. 4,211,801 describes separation in an organic solvent, whereby of oat flour free of oat gum is prepared, but separation of starch from it is not described.
Separation and purification of starch are described in detail in the European Patent Application 89.200321.1, and in the corresponding Finnish application 900508. The method is based on wet milling in a solution containing sulfur dioxide, and subsequent sieving and hydrocyclone operations. For achieving the separation of starch from cell walls, enzymes degrading cell wall components such as cellulose, hemicellulose and .beta.-glucan are used. The particle size distribution of the starch obtained using this method is according to the patent specification within limits 4-15 .mu.m.
Enzymatic steps are also included in processes where starch is further hydrolysed to maltodextrin, as in the U.S. Pat. Nos. 4,996,063 and 5,082,673. These methods are, however, not intended for separation of unhydrolyzed starch. In a method according to the Canadian Patent 1,179,189, hull-less oat groats are soaked in sulfur dioxide containing water for 24-28 hours at 50.degree. C. The inherent enzymatic activity of the kernel decomposes cell wall material, and the endosperm containing starch is separated from the bran by squeezing. In this method, too, no description of starch separation is included.
In the method described in the Finnish Patent 84,775 and in the corresponding European Patent 379,499, oat endosperm is treated in sodium hydroxide solution to decrease the protein content. The patent specification does not give information on the particle size distribution.